It is known to use pistons for large bore internal combustion engines. Typically, pistons have essentially two functional parts, a crown and a skirt. There are two basic designs for large bore pistons, articulating and non-articulating. Articulating pistons have separate skirts that can pivot about a wrist pin relative to the crown. In comparison, non-articulating pistons have skirts that are integral with the crown. It is also known to divide the crown into upper and lower portions that are subsequently fastened together to form a three-piece piston. The lower crown includes a pair of pin bosses which each have a pin bore to receive the wrist pin.
Some non-articulating pistons are called composite pistons because they have a steel upper crown connected to an aluminum, or other light alloy, lower crown to reduce weight. Steel is employed in the upper crown to maintain an adequate degree of strength in the head region, which is exposed to very high combustion temperatures and pressures. The steel upper crown is often connected to the aluminum lower crown using precision bolts.
The hybrid metal composition also sacrifices the overall strength of the piston. During operation, pistons experience two separate loads. First, the crown experiences a vertical load from the combustion of the gas gathered in the combustion bowl at the top of the upper crown. It is important to select a material that is strong enough to withstand the combustion load, for example, steel. Next, the skirt receives a side load from the mechanical motion of the crankshaft. In non-articulated pistons, the skirt is integral with the lower crown therefore, both loads are imposed on the entire piston. As a result, the movement and performance of the crown interferes with the movement of the skirt.
Also, hybrid steel/aluminum pistons typically have piston ring grooves located in the side walls of the upper crown because aluminum side walls are not strong enough. If the ring grooves are located in the aluminum side walls, they must be reinforced with a stronger metal alloy to accommodate the vertical load experienced during combustion.
The combined load on the crown and skirt also causes severe fretting wear in known bolted, non-articulated pistons, and reduces service life. Fretting wear occurs in generally stationary joints when very minor amounts of relative movement create microwelding between components, such as between an upper crown and a lower crown. Microwelds are formed and then broken in successive movements between the crown parts. Further, increased fretting wear occurs at the steel/aluminum interface increasing the amount of contamination particles in precision engines and reducing service life of the piston and engine.
The wrist pin is mounted in the pin bores of both pin bosses, constituting a bearing, and must be adequately supported against the piston. Thus the pin boss is the pivot of the piston, the wrist pin and connecting rod assembly. In highly loaded piston applications, a particularly complicated design of the pin boss is needed when aluminum or other light metal alloys are used for the pin boss material. Incipient cracks can occur in the wrist pin if the maximum allowable specific pressure is exceeded. To avoid such cracks in the wrist pin, it is known to profile the pin bore to relieve the stresses caused by deformation of the wrist pin. In a non-articulated piston with an aluminum lower crown, the thickness of the walls around the pin bores and precision profiling of the pin bore itself are necessary to overcome the weakness of the aluminum material.
One alternative for attaching an upper crown to a lower crown is friction welding. However, pistons of 250 mm diameters and more are very heavy and are relatively low volume items. In addition, many friction welding machines do not have enough radial clearance to assemble large diameter pistons. Thus, it is not usually economically feasible to invest in larger friction welding equipment to assemble low volume, large diameter pistons.